Electrochromic (EC) windows and other optically switchable windows typically need power to induce and maintain an optical change. In an EC window, for example, voltage/current must be applied to the transparent conductive sheets sandwiching an optically active layer to change the EC window from a bleached state to a tinted state. A small voltage/current may be needed to hold the EC window in the tinted state. The voltage/current is applied by bus bars electrically connected to the conductive sheets. The bus bars are electrically connected to a power source that supplies the power needed for the optical changes. Window controllers are used to adjust the power applied to the EC windows to control its optical changes.
Buildings equipped with EC windows do not typically have mechanical shades since that would be considered an unnecessary under normal circumstances. If a power outage were to occur in such a building, the window controller controlling the tinting levels of the EC windows could lose power along with the air conditioning (AC) and heating systems. Under these circumstances, power would not be available to tint any clear windows or clear any tinted windows. The EC windows would “float” (i.e. be released of applied voltage/current) and would eventually lose their tint due to current leakage. If conditions outside the EC windows are hot and sunny, solar radiation and its associated heat would be allowed to penetrate the rooms. If the AC is not working as well, the internal room temperatures could become uncomfortably warm for its occupants. On the other hand, if conditions are cold and sunny, since power is not available to clear the windows quickly, the windows would instead slowly lose their tint. During this time, the tinted windows would block the solar radiation and prevent harvesting of the solar energy to help heat the rooms where the heating systems may not be available. In this case, occupants may be subjected to uncomfortably cold temperatures. Moreover, during a power outage, the local window controllers at the EC windows may not have a working power supply and occupants could lose their ability to control their EC windows to make their rooms comfortable, for example, from glare. Although certain buildings are equipped with a backup power supply or an uninterruptable power supply (UPS) that automatically switches on in a power outage, their energy stores may be quickly depleted if the EC windows and other systems in the building continue to run under normal operation.